In the electrowinning of copper, electrolytes are utilized in which the arsenic content tends to increase with time. A copper electrowinning electrolyte can contain, for example, high arsenic contents of as much as 8 grams per liter of arsenic.
The arsenic in the copper electrowinning electrolyte poses some significant problems. Firstly, the arsenic tends to precipitate upon the inert lead anodes which are used in the process.
In addition, there is a tendency toward arsine formation with its significant environmental hazard.
It has been proposed to remove arsenic from electrowinning electrolyte after the electrodeposition of copper and nickel therefrom by the formation of an arsenic precipitate, leaving the resulting sulfuric acid solution substantially free from the arsenic. Such processes, however, are disadvantageous, in that the treatment used to precipitate the arsenic at high arsenic concentrations in the copper electrolyte can result in the formation of the toxic arsine gas (see Ullmanns Encyklopadie der Technischen Chemie, 3. Auflage, Urban & Schwarzenberg, Munchen, Berlin, 1961, pages 213 and 197).
To avoid these problems it has been proposed, for example in British Pat. No. 1,551,023, to remove the arsenic from the copper electrowinning electrolyte by an extraction process utilizing an organic solvent. The extracting agent proposed in this patent is tributylphosphate (TBP) a compound which, when present in an amount of 50% by weight in the organic extraction phase, is able to recover say 4.5% of the arsenic from a copper electrowinning electrolyte. The process uses liquid/liquid extraction and the arsenic can be recovered by treating the extraction mixture with an aqueous solution.
While the extraction process has proved to be a marked improvement over earlier methods of removing arsenic from such electrolytes, the low recovery of arsenic has left much to be desired.